1. Field of the Invention
The present invention relates generally to broadcast receivers, and more particularly to broadband tuners utilizing a local oscillator.
2. Related Art
Broadcast satellites transmit signals over multiple channels across a broadband spectrum. These satellites may be combined with receiving systems to create a direct broadcast satellite system (DBS). A current example of such a system is Digital Satellite Service / Direct Video Broadcast (DSS/DVB), which broadcasts over the range 12 to 14 GHz. A DSS/DVB receiving system generally includes an antenna, a low-noise block converter (LNB) and a direct down-conversion broadband receiver. The direct down-conversion receiver comprises a tuner, an analog-to-digital converter and a baseband demodulator. The LNB converts the 12-14 GHz band signal received by the antenna into a 950-2150 MHz band signal. The tuner directly converts this 950-2150 MHz band down to baseband, which consists of in-phase (I) and quadrature (Q) components. These I and Q components are then transformed into digital data by the analog-to-digital converter and demodulated by the baseband demodulator to decode the video and audio being broadcast.
The tuners in such systems typically utilize mixers to perform the down-conversion from 950-2150 MHz to I and Q components. These mixers require a local oscillator, which typically consists of a frequency synthesizer with a step size equal to the receive channel spacing. Such local oscillators are often implemented using a frequency synthesizer and a voltage controlled oscillator.
The signal from this local oscillator drives the mixers. Commonly, the local oscillator operates at the same frequency as the carrier frequency of the selected channel.
However, some local oscillators are designed to operate at approximately twice the frequency of the carrier signal. This is done to avoid the many problems, such as cross modulation and direct current (DC) offset, which arise when the local oscillator operates at exactly the same frequency as the carrier. Thus in these systems, the local oscillator signal is passed through a divider before being fed to the mixers. Additionally, in some of these designs, the phase shift required to obtain the Q component is also performed within the divider. However, even in these types of systems, a strong RF signal can modulate the local oscillator signal, thus causing tuner degradation.
Typically, the local oscillator consists of two external voltage-control oscillators (VCOs). The external VCOs are typically off-chip tank circuits, residing inside the set-top box and using the common transformer as a power source. These tank circuits commonly include four inductors and two hyper-abrupt varactor diodes. The external hyper-abrupt varactors commonly use 30 volts of tuning range, and have a ten-to-one capacitance, thus providing the large tuning range needed for broadband signals.
The problems with this traditional approach to broadband tuners are multiple. First, when the tank components are off-chip, the inductors will radiate like an antenna. Thus, these traditional tuners create high frequency radiation, which must be dealt with to avoid interference with other components on the board. Second, the common solution to the unwanted radiation is to add shielding. This solution makes the tank components larger and more difficult to integrate onto the board, thus requiring more board space and increasing the cost of producing the tuner. Third, the traditional off-chip tank circuits include 30 V hyper-abrupt varactor diodes, thus requiring the set-top box to include a 30 V tap off the transformer. This also increases the cost of producing the tuner by taking up valuable space inside the set-top box.
Integrated wideband tuners have been reported using multiple wideband RC oscillators. While these tuners reportedly solve the problems of coupling between the radio frequency and the local oscillator signals, DC offset and the need for a 30 V power supply, they do not solve the many other problems addressed by the present invention. In addition, they require a complicated multi-loop architecture to achieve desired phase-noise performance. This complicated multi-loop architecture is undesirable because of the added expense and difficulty in system-board design it creates.
Therefore, what is needed is a broadband tuner that does not require radiation shielding, generates less heat, is smaller, more robust, less expensive and easier to integrate into the system board.
In accordance with the purpose of the present invention, as broadly described herein, there is provided a fully integrated single-loop frequency synthesizer, which can serve as a local oscillator for a broadband tuner, thus allowing the creation of a single-chip solution for broadband applications.
A feature of the present invention is the integration of the tank circuits into the tuner chip through the use of multiple VCOs comprising narrow-tuning range varactors. This integration reduces the number of components required to make a broadband tuner. In this fashion, the present invention reduces the cost of producing a broadband tuner and makes system-board design easier.
An additional feature of the present invention is the elimination of the high frequency radiation associated with the inductors commonly used in broadband tuners. When the VCOs are on the chip, this radiation is eliminated, thus removing the need for shielding. In this fashion, the present invention further reduces the cost of producing a broadband tuner and makes system-board design easier.
According to a preferred embodiment of the present invention, a single-loop frequency synthesizer is fully integrated into a tuner chip to serve as the local oscillator, which feeds on-chip down-converting mixers. This integration is accomplished by combining a phase-locked loop (PLL) with multiple VCOs comprising narrow-tuning range varactors. This combination of a PLL and multiple VCOs enables the use of a method to build up coverage of the full broadband input (950-2150 MHz). In this fashion, the resulting broadband tuner is made smaller, cheaper and easier to use in system-board design.
In a preferred embodiment of the present invention, drift in the VCOs caused by heat is overcome by careful engineering. Each VCO is designed to overlap with its adjacent VCO. The size of this overlap, or breakpoint, is made large enough that the local oscillator can tolerate temperature drift in the VCOs. In this fashion, any drift in the VCOs caused by heat is prevented from adversely affecting the tuner.
According to another preferred embodiment of the present invention, careful engineering and a calibration method overcome the significant initial tolerance problems associated with the VCOs. A sufficient number of VCOs are provided such that whatever the initial tolerance shift, the full broadband spectrum can still be covered. This is possible because each on-chip VCO""s initial tolerance shifts in a similar fashion with all the others. Thus, no frequency gasps between VCOs can arise. At start up, the VCOs are calibrated to determine the size and location of the breakpoints. In this fashion, the initial tolerance is effectively zeroed out.
According to a preferred embodiment of the present invention, the problem of coupling between the local oscillator (LO) signal and the incoming radio frequency (RF) signal is mediated by integrating the tank circuits into the chip and by using a 2/4 divider. RF to LO coupling is reduced by putting the VCOs, including their inductors, on the chip. Additionally, the VCOs in the local oscillator are designed to operate at a much higher frequency than the RF signal. During the calibration of the VCOs, it can be determined whether to divide the LO signal by two or by four before mixing the LO signal with the RF signal. This on-the-fly determination adds flexibility to the design of the VCOs, thereby making the resulting tuner more robust.
According to a preferred embodiment of the present invention, the initial calibration of the VCOs is accomplished by utilizing the lock detect output of the phase-locked loop and a binary search algorithm. For each VCO, the upper and lower edge is determined. The lower edge is the lowest frequency to which that VCO can be tuned, as identified by the PLL. The upper edge is the highest frequency to which that VCO can be tuned, as identified by the PLL. With this upper and lower edge information a look-up table is created such that, for each RX frequency, the appropriate VCO and VCO divider to use are known. In this fashion, the calibration of the VCOs is accomplished, and the local oscillator can thus be implemented on one tuner chip, as a single-loop frequency synthesizer using multiple on-chip VCOs. Thus, the resulting broadband tuner is made smaller, less expensive, more robust and easier to use by system-board designers.